Safety-relevant functions, such as electrical brakes, are being increasingly operated electrically in vehicles. In order to ensure that the serviceability of such safety-relevant components is guaranteed, they must be supplied with electrical power all the time. In order to achieve this, it is known for two energy sources to be provided in a vehicle, namely a battery and a generator.
Since the life of a conventional lead-acid battery is generally shorter than the life of a vehicle, it is possible for only one of the two energy sources to still be serviceable. This situation, in which a battery is discharged or defective and is thus unserviceable can result in damage to the battery or overloading of the second energy source, the generator. This can lead to the supply to the loads no longer being ensured. Furthermore, the vehicle may change to a safety-critical state. For this reasons, measurements of the battery's buffering effect are required which on the one hand warn the driver and on the other hand also keep the vehicle power supply system, and hence the vehicle, in an operationally safe state to ensure that loads which are relevant for operation and in particular those which are relevant to safety, such as engine electronics, electrical brakes, are not switched off. In addition to ensuring that as little current as possible is drawn, a minimum voltage level must be maintained in order to guarantee the operability of safety-relevant devices such as these. This can normally be ensured by means of the generator. However, if the battery has been deep-discharged or is defective in some other way, then it can no longer provide a buffering effect. This may possibly lead to the generator being de-energized by a load change during operation and the vehicle power supply system voltage suddenly collapsing, so that it is no longer possible to ensure that the safety relevant devices are operated.
In addition to ensuring that as little current as possible is drawn, a minimum voltage level must be maintained in order to guarantee the operability of safety-relevant devices such as these. This can normally be ensured by means of the generator. However, if the battery has been deep-discharged or is defective in some other way, then it can no longer provide a buffering effect. This may possibly lead to the generator being de-energized by a load change during operation and the vehicle power supply system voltage suddenly collapsing, so that it is no longer possible to ensure that the safety relevant devices are operated.
A method for determining the buffering effect of a battery for providing a voltage for a power supply system for a vehicle is known from DE 199 44 517 A1. In this case, the voltage ripple is detected and determined, and the maintenance of a limit value which can be predetermined is monitored using a monitoring unit.
Furthermore, a method for testing the quality of a battery is known from Japanese Laid-Open Specification JP 03-249 582 A. In this case, a constant AC source is used as a load for the battery to be tested, and the voltage level which then occurs is determined. The quality of the battery is then assessed on the basis of the resultant voltage levels. However, owing to the use of the constant AC source, an arrangement such as this cannot be used in a vehicle. Furthermore, the use of the constant AC source represents an extra signal being fed in, and this is complex.
The invention is thus based on the object of specifying a method for determining the buffering effect of a battery, which allows the serviceability of a battery to be identified as easily and reliably as possible. A further aim is to specify a particularly simple arrangement for determining the buffering effect of the battery.